It is known in the art for a communication system (possibly including one or more wireless or radio access networks) to provide group or conference calling. It is also known in the art to use SIP (Session Initiation Protocol) to set up a call between two or more call participants when at least one is a subscriber to a communication network (and so communicates with the other participants over a communication path that includes the radio access network of the communication network). Additionally, it is known in the art to use a SIP INVITE method (a message or in other words a communication) having a Call-Info header containing a URL (Uniform Resource Locator) referring to (a document or file providing) supplementary call information. In addition, a SIP INVITE method can also have an Alerting-Info header containing a URL referring to a ringing tone or other headers used for including additional information about the session (e.g. the subject to be discussed during the call being set up). The supplementary call information or ringing tone must be fetched by the callee by some additional methods, using e.g. HTTP (hypertext transfer protocol).
What the prior art does not teach, however, is for a mobile caller to provide, as part of call set up signalling, information about a proposed conference call, such as that the call being set up is to be a conference call and who the invited participants are. It would be advantageous to use SIP signalling for call set up to invite subscribers reachable by an originating subscriber to participate in a group or conference call (and so to identify the call as a proposed conference call), and also to use SIP signalling during call set up to provide other services in connection with setting up a conference call, such as to indicate the other invited participants.